Methods for scanning an object under examination with a computed tomography system (CT system) are generally known. For example, circular scannings, sequential circular scannings with advance or helical scanning (so-called “spiral scanning”) are used for this. With these types of scanning, at least one X-ray source and at least one oppositely located detector record absorption data from the object under examination from different angles and these absorption data or projection data collected in this way are converted by way of corresponding reconstruction methods to form sectional views or three-dimensional volume image data through the object under examination. Computed tomography systems usually use detector systems embodied as a detector array comprising a plurality of X-ray detector elements arranged in rows and columns. Here, the detector system is generally embodied as a partially circular detector, which is arranged opposite to the X-ray source on a so-called gantry and rotates with the gantry or X-ray source. There are also computed tomography systems having a complete detector circuit, wherein the individual X-ray detector elements are read according to the position of the X-ray source.
For the reconstruction of computed tomography images from X-ray CT data records of a computed tomography device (CT device), that is from the acquired projection data, a so-called Filtered Back Projection (FBP) is nowadays employed as the standard method.
In present-day dual-source CT systems (that is CT systems with two or more focus/detector systems), and also in single-source CT systems, with an increasing width of the detector, the greater the significance attached to the scattered radiation in the feed direction, i.e. parallel to the axis of rotation of the X-ray system, (generally referred to as the “z-direction”, in which the detector columns—also known as detector channels—extend). In the dual-source CT devices on the market hitherto, attempts are made to compensate the negative influence of the scattered radiation, in particular the cross scatter, on the quality of an image by means of a scattered radiation correction. In principle, with scattered radiation, a differentiation is made between forward scatter and cross scatter.
For detector widths from 4 cm or for quantitative methods, such as those used in particular with, for example, dual energy CT measurements, the scattered radiation correction is based on a measurement of the cross scatter by sensors attached in the z-direction outside the penumbra of the cone beam of the X-ray tube. Typically, there is a row of scattered radiation sensors along both sides of the detector. These scattered radiation sensors can, on the one hand, be conventional detector elements placed outside the useful fan of the X-ray beam (that is outside the detector array used to detect the primary radiation). In some CT systems, on the other hand, dedicated scattered radiation sensors are used outside the main detector. This means that scattered radiation sensors are disposed along the detector on both sides, generally, for each detector module, there is one scattered radiation sensor in the z-direction in front of the main detector and one scattered radiation sensor in the z-direction after the main detector, wherein one detector module in each case encompasses a plurality of detector columns arranged side by side extending in the z-direction.
For an ideal focal spot (e.g. with a rectangular intensity profile), the diaphragm at the tube side can be designed so that only scattered radiation that occurs in the objects to be measured arrives at the scattered radiation sensors and can be measured. However, in reality, the focal spot is surrounded by a low-intensity aureole, a so-called spatially extended halo. This halo is in principle present in all X-ray emitters in which electrons are decelerated in the anode. Unlike the case with the useful focus, the diaphragm close to the tube cannot keep the radiation emerging from this extended halo, the so-called extra-focal radiation completely away from the scattered radiation sensors.
Since this extra-focal radiation superimposed on the scattered radiation also traverses the object to be measured and is ultimately measured in the scattered radiation sensors, this results in unwanted tomography of the regions adjacent to the useful fan. This means that the scattered radiation measured values contain further additional intensities due to the additional tomographic data. Since the scattered radiation correction substantially consists of a subtraction of measured or calculated scattered radiation outside the useful fan, a contrast reversal of these incorrectly additionally measured structures takes place in the reconstructed image data so that a sort of “ghost image” forms. With increasing detector widths in the z-direction, these “ghost image” phenomena represent an increasing problem in particular for dual-source CT systems, since information from ever more remote body regions are projected onto the wrong position.
One reason for this is that with an increasing detector width in the z-direction of the detector, the diaphragm close to the tube also has to have a further aperture in the z-direction. As a result, correspondingly more extra-focal radiation reaches the sensors outside the actual useful fan.
A further parameter influencing the amplitude of the superimposed extra-focal radiation in measurements of scattered radiation with scattered radiation sensors is the distance from the sensor to the penumbra of the focus. For example, the smaller the distance between the sensor and the penumbra, the more extra-focal radiation falls on the sensor. Therefore, the sensors for the measurement of scattered radiation are usually mounted at a sufficient distance from the penumbra. However, with an increasing detector width in the z-direction, in clinical CT, frequently the entire mounting space available to the detector and the scattered radiation sensors plays a role so that, for this reason, a greater distance between the sensors and the detectors would not be favorable.